Report: Mass whale deaths off the coast of Scotland were caused by Royal Navy bomb explosions

A beached humpback whale. Wikimedia Commons Four large bombs exploded underwater by the Royal Navy were to blame for a mass stranding which killed 19 pilot whales on the north coast of Scotland in 2011, government scientists have concluded.

A long-delayed report released on Wednesday by the Department for Environment Food and Rural Affairs says that the noise from the explosions could have damaged the hearing and navigational abilities of the whales, causing them to beach and die.

On 22 July 2011, 70 long-finned pilot whales swam into the Kyle of Durness, a shallow tidal inlet east of Cape Wrath, Europe's largest live bombing range. Despite attempts to herd them back out to sea, 39 were left stranded by the tide.

Concerted efforts by expert teams and local people managed to refloat 20, but 19 ended up dead. It was one of the largest mass strandings in recent years, and it prompted a government-funded investigation by 12 scientists from laboratories across the UK.

Their report reveals that three 1,000-pound bombs were detonated in the sea nearby by the Royal Navy's Northern Diving Group in the 24 hours before the whales were stranded. A fourth 250-pound bomb was exploded after stranding began.

The bombs were left over from military exercises in which planes target Garvie Island, a small rocky outcrop 4.5km from the Kyle of Durness. Some bombs miss the island, fail to detonate and sink to the seabed, where they have to be located and disposed of for safety reasons.

"The magnitude, frequency and proximity of the multiple detonations in the day prior to the stranding, and the single high-order detonation shortly after the beginning of the mass stranding, were plausible sources of significant disturbance to any neighbouring marine mammals," the report says.

A Royal Navy ship. David Jenkins/Royal Navy The three initial explosions could have had a "significant detrimental effect on the hearing and therefore navigational competence of any cetaceans in proximity," it adds. The fourth bomb "might have served to drive the animals further inland".

Loud noises can damage the hair cells in the ears of whales vital for detecting pressure changes, leaving them "functionally deaf", the report points out. "Long-finned pilot whales are known to follow other members of the pod and appear to spook relatively easily."

It criticises the Royal Navy's visual checks for whales before bombs are exploded as "insufficient", and recommends improved monitoring. It also highlights the routine use of devices elsewhere in the world that burn out rather than detonate bombs.

"Given the potential damage to marine life from the high-order explosions of conventional disposal techniques, it is questionable why this method has not been used routinely in the past," the report says.

The lead author of the report, Andrew Brownlow from Scottish Marine Animal Stranding Scheme at Scotland's Rural College (SRUC) in Inverness, was pleased that it had "finally" been published. It was hard to be definitive about the causes of mass strandings, he said.

"However we have suggested mitigation strategies which will hopefully reduce the plausible risk from these types of high-energy detonations on marine life. It is hoped they will be taken on board."

According to Sarah Dolman, Northeast Atlantic programme manager for Whale and Dolphin Conservation, it was "no coincidence" that the whales were stranded hours after the bombs were exploded.

She said: "Why has it taken four years to publish the report and what measures have the Ministry of Defence (MoD) put in place to evaluate and minimise the impacts of detonations around Garvie Island, to ensure that it adequately protects whales and dolphins since then?"

The MoD said that it accepted the findings of the report. "It identified a number of possible factors that may have influenced events, one of which was the detonation of underwater explosives," said a spokesman.

"The recommendations will be considered by the MoD and implemented where appropriate. Additional mitigation has already been put in place during munitions disposal activities conducted since 2011."